The present invention relates to series wound motors, also referred to as universal motors. In particular, the invention relates to an electric braking system for such motors.
In certain appliances and tools which utilize series motors, it is desireable to stop the movable working member in a very short period of time rather than letting the working member "coast" to a stop upon deenergization of the motor. For example, in some tools, such as circular saws, unbraked stopping time may be rather long, such as 7 to 12 seconds, due to inertia of the motor armature, the gearing, the shaft and the circular saw blade. It is often advantageous to stop the circular saw blade or other rotating working member in a much shorter time period, say 2 to 3 seconds.
Mechanical brake systems have been provided for tools and appliances of the type under consideration. These mechanical brakes suffer from a number of disadvantages. Mechanical brakes are bulky thus adding undesired weight to the tool or appliance. Further, these mechanical brakes are expensive to manufacture and require frequent maintenance.
The prior art also includes electric brake systems for stopping the rotating working member of a tool or appliance. The most common form of electric brake system employs a regenerative principle. When a regenerative circuit is used, release of the "on-off" switch of the tool or appliance creates a reversal of polarity effective between the field windings and the armature windings. In the presence of the residual magnetic field in the stator, regenerative direct current reversal is produced through the armature and field windings. This current flow is in the direction of reversing armature rotation and its effect is to slow and stop the armature. Thus, rotation is stopped more quickly than if the tool is allowed to "coast" to a stop relying only on friction and windage. Since regenerative brake circuits rely on current flow between the brushes and the commutator, it is necessary to provide for very good contact between the brushes and the commutator to ensure reliability and effectiveness of the braking circuit.
The carbon brushes used in series motors are normally supported in a brushholder which is in the form of a parallelopiped open at one end thereof. A coil spring in the brushholder urges or biases the brush into engagement with the rotating commutator. It is known in the art to provide such brushholders with mechanical means for limiting the amount of travel of the brush and to cause the brush to be pushed away from the commutator when the brush has been worn a predetermined amount. The purpose of this particular type of brushholder is to prevent the brush from wearing to an extent that the remaining portion of the brush might become dislodged from the brushholder. In that event, it is likely that the coil spring will contact the rotating armature thus causing not only shorting but damage to the commutator as well. Brushholders with means for limiting the amount of brush movement also avoid a condition where the coil spring would stop in the brushholder and not urge or bias the brush into engagement with the rotating commutator. This situation would cause extra arcing and generate damage to the commutator. Although these forms of brushholders have been available for many, many years, they have not been employed in braking circuits for separating at least one brush from the commutator prior to the time the brush might wear to an extent that reverse current could not be properly conducted to achieve the regenerative braking effect.